Mr Bersani won the first round on 25 November, with 45% of the vote to Mr Renzi's 35.5%. Partial results on Sunday saw him polling more than 60%.

Analysis

By Alan Johnston BBC News, Rome

The 37-year-old mayor of Florence, Matteo Renzi, presented himself as the face of a new and more dynamic generation. He talked of the need for his party to push aside its incumbent leader, Mr Bersani, who is in his sixties.

But the older man seems to have held on easily. He'd argued that his lifetime of experience in politics and government were what the country needed. And he will now relish the prospect of preparing his centre-left party for the general election that's expected in the spring.

His opponents on the right are in disarray. The former prime minister, Silvio Berlusconi, is still undecided as to whether he'll lead his party into the election. And Mr Bersani's faction has a healthy lead in the polls. If that holds until election day, he could well find himself in a position to form a government.

Mr Renzi tweeted that "it was right to try" and later congratulated his rival at a rally in Florence.

"They won and we did not," he said.

In his victory speech, Mr Bersani said job creation would be a priority as he put together a programme of policies for the electorate.

He would ensure there was room for younger members of the party to express themselves as they prepared for the challenges ahead, he added.

The BBC's Alan Johnston in Rome says the two men are very different characters, with tieless, youthful Mr Renzi painting himself as the voice of a new generation which wants to sweep away the entire class of older politicians.

By contrast, adds our correspondent, 61-year-old Mr Bersani has argued that experience is what the country needs.

Speculation has been mounting as to whether centre-right leader Silvio Berlusconi would run for a fourth term in office next year, with the former prime minister suggesting last week that he was thinking about returning to politics.

The billionaire tycoon's record has been tarnished by sex and political scandals, and he resigned as prime minister in November 2011, after MPs approved an austerity deal to help curb the debt crisis threatening the eurozone.